Mouse spleen cells were modified with trinitrobenezene sulfonate (TNP), and the TNP-self modified cells were tested in two different ways. First, several monoclonal antibodies (mAbs) specific for class I H-2 antigens were tested for binding to TNP-modified spleen cells. Second, the modified cells were used as stimulator and target cells for in vitro tests for cytotoxic T lymphocyte (CTL) responses to TNP-self. A number of anti-H-2Kk antibodies not mAbs of other specificities exhibited enhanced binding to cells that normally express Kk. Furthermore, these same mAbs bound to H-2b cells modified with TNP. These results parallel the patterns of preferential CTL recognition in H-2k mice and of crossreactive CTL in H-2b mouse strains.